The present invention relates to apparatus for manipulating sheets, especially folded sheets which can constitute component parts of signatures or the like. More particularly, the invention relates to improvements in apparatus of the type wherein a conveyor withdraws successive folded sheets from a source of supply and advances the withdrawn sheets into a path wherein the folded back of the sheet is located behind the two front edges and wherein two rotary spreading or opening elements flank the path and are provided with jaws, grippers or analogous means for engaging the respective front edges and for moving them apart while the sheet advances along its path.
Apparatus of the above outlined type are often used in bookbinding and like machines. Reference may be had to commonly owned U.S. Pat. No. 4,085,927 granted Apr. 25, 1978 to Hans Muller and to commonly owned U.S. Pat. No. 4,299,378 granted Nov. 10, 1981 to Hans Muller. The arrangement is normally such that the spreading elements rotate at an average speed, that they engage and begin to open a sheet when they assume predetermined first angular positions, and that they release the opened sheet in second angular positions so that the sheet can descend onto the upper reach of a removing device, such as a chain conveyor or the like. The trend in the design of machines utilizing such apparatus is to greatly increase their output; however, this should not affect the reliability of withdrawal of successive sheets from the source and/or the reliability of the spreading or opening operation. Thus, the conveyor which withdraws the sheets from the source of supply must reliably engage successive sheets preparatory to advancement of the thus engaged sheets into the aforementioned path, and the jaws of the spreading elements must engage the respective front edges of successive sheets with an equally high degree of reliability and reproducibility. Certain presently known proposals for enhancing the reliability of engagement between successive sheets and the withdrawing conveyor, as well as between successive sheets and the spreading elements, are disclosed, for example, in Swiss Pat. Nos. 374,968 and 617,905, in British Pat. No. 901,816 and in the aforementioned U.S. Pat. No. 4,299,378 to Muller.
The speed of transport of successive folded sheets in heretofore known machines cannot be increased at will without risking at least some misalignment of sheets which are deposited on the removing conveyor. This is due to the fact that the resistance of air to advancement of opened sheets toward the removing conveyor increases with the square of increasing speed of advancement of successive sheets along their path. In order to reduce the likelihood of misalignment of sheets which are transported at an elevated speed, the aforementioned U.S. Pat. No. 4,085,927 to Muller discloses the utilization of nozzles which blow compressed air against the outer sides of opened sheets so as to accelerate the sheets on their way toward the removing conveyor as well as to ensure that the sheets are properly guided during travel toward such conveyor. It has been found that the accelerating and guiding action of nozzles is rather limited, i.e., they cannot do the job when the speed of transport of successive folded sheets is increased beyond a certain threshold value.